because of instinct, which is simply an inherited habit. Rivers may be safer than the ocean for their young. They are worn and exhausted by the journey, and never survive to lay eggs the second time.
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Fig. 228.—A Shark (Acanthias vulgaris).
The air bladder is developed from the food tube in the embryo fish, and is homologous with lungs in the higher vertebrates. Are their functions the same?
Fish that feed on flesh have a short intestine. Those that eat plants have a long intestine. Which kind of food is more quickly digested?
There are mucous glands in the skin of a fish which supply a secretion to facilitate movement through the water; hence a freshly caught fish, before the secretion has dried, feels very slippery.
The air bladder, although homologous to lungs, is not a breathing organ in common fishes. It is filled by the formation of gases from the blood, and can be made smaller by the contraction of muscles along the sides of the body; this causes the fish to sink. In the gar and other ganoids, the air bladder contains blood vessels, is connected with the gullet, and is used in breathing. Organs serving the same purpose in different animals are said to be analogous. To what in man are the gills of the fish analogous? Organs having a like position and origin are said to be homologous. The air bladders of a fish are homologous with the lungs of man; but since they have not the same use they are not analogous.
